The present invention relates to organic solvent soluble aromatic polyimides containing the hexafluoroisopropylidine or trifluoroethylidene linking group and to a method for rendering such polyimides substantially insoluble in organic solvents.
Polyimide condensation products of 2,2-bis (4-amino-phenyl)hexafluoropropane and aromatic dianhydrides are described in U.S. Pat. No. 3,356,648. These polyimides may be produced by first preparing a polyamic acid by reacting the diamine and a suitable dianhydride such as pyromellitic dianhydride in a solvent for at least one of the reactants to produce the polyamic acid followed by conversion of the polyamic acid to the polyimide. For the preparation of films, the polyamic acid solution is applied to a substrate and heated at temperature of up to about 300.degree. C. for a period of several hours to imidize the polyamic acid. Such films exhibit high tensile properties, high thermooxidative stability and low dielectric properties which render them extremely useful as printed circuit backings, insulating layers for wire and cable assemblies, protective layers for solar cells and like applications. Such polyimides are also suitable in the fabrication of reinforced composites, molded parts, fibers and the like.
Many polyimides containing the hexafluoroisopropylidene linkage derived from aromatic diamines and aromatic dianhydrides (with the exception of pyromellitic dianhydride) are soluble at room temperature in many organic solvents such as chlorinated solvent, n-methyl-pyrrolidone, acetone, methyl ethyl ketone, diglyme, gamma-butyrolactone, propylene glycol methyl ester and like materials. This property of solubility permits the direct application of solutions of the finished polyimide to a substrate by spin coating, spray coating and brush coating methods. It is most advantageous for the fabricator to form a coating or laminate using the finished polyimide rather than the intermediate polyamic acid solution because the polyamic acid solutions have poor storage stability and require refrigeration, and also because the evolution of volatiles such as water during the in-situ heat imidization step can lead to non-uniformity of the surface and cross section of the film or article.
However the advantage of solubility can be a detriment in certain applications where the surface of the polyimide must be cleaned or otherwise treated with organic solvents after application to a substrate, or where composites and shaped articles come into contact with solvents or fuels. For example, polyimides are commonly used in flexible printed circuitry applications wherein the film is laminated to a backing such as copper, then coated with a photoresist, exposed and etched. Often times the polyimide surface must first be cleaned with organic solvents to remove dust and other impurities prior to the application of the resists, such as disclosed in U.S. Pat. No. 3,833,436. Such a treatment can result in a cracking or partial dissolution of the soluble polyimide which will detract from the effectiveness of the polyimide as an insulating or dielectric layer. In addition, composite articles or shaped parts used in the aircraft industry (fuel tank liners, for example) must be inert to solvents used in their cleaning and to fuels with which they may come in contact.
Accordingly, it is an object of this invention to provide a method for rendering coatings, films composites and other articles based on solvent soluble polyimides insoluble after application to a substrate surface or after shaping.
It is also an object of this invention to improve the adhesion of polyimide films to substrates such as metals and silicon wafers.